1. Field of the Invention.
The present invention relates, in general, to a method of fabricating a multi-layered printed circuit board (PCB) for optical waveguides and, in particular, to a method of fabricating a multi-layered PCB, in which the optical waveguide component is inserted into the PCB in such a way that a prepreg adhesive does not block an optical signal entrance of the optical waveguide.
2. Description of the Prior Art
As well known to those skilled in the art, PCBs are circuit boards whereby various electronic elements are densely mounted on a flat plate consisting of a phenol or epoxy resin and a circuit, electrically connecting the electronic elements to each other, is fixed on a surface of the resin.
A method of fabricating such a PCB includes attaching a copper foil to one side of the phenol or epoxy resin insulating substrate, etching the resulting substrate to form a desired circuit pattern (a portion other than the circuit pattern is eroded and removed), and piercing holes for receiving the electronic elements on the etched substrate.
Further, the PCBs are classified into three types according to the number of patterned layers constituting the PCB: single-sided PCBs, double-sided PCBs, and multi-layered PCBs. The higher the number of the patterned layers is, the higher the number of electronic elements mounted on the patterned layer will be. Of the three kinds of PCBs, accordingly, the multi-layered PCBs are applied to sophisticated products. On the other hand, the single-sided PCBs mostly use phenol substrates and are applied to products provided with simple circuitry such as radios, telephones, and other such simple-structured instruments. The double-sided PCBs mostly use epoxy resin substrates and are applied to products provided with relatively complicated circuitry such as color TVs, VTRs, and facsimile telegraphs. In addition, the multi-layered PCBs as described above are applied to sophisticated products such as 32-bit computers or computers with even higher performance, electronic switchboards, and high performance telecommunication equipment, and each comprise three or more layers with a conductive pattern in such a way that each insulating layer is inserted between adjacent patterned layers.
Furthermore, when there is a need for a portable or removable circuit substrate such as in automation equipment and camcorders, or when the circuit substrate constituting the electronic products is needed to be bent, a flexible PCB is used instead of a rigid PCB.
A conventional PCB comprises a copper plate on which a circuit pattern is formed, and an inner and an outer layer. However, recently, an electro-optical circuit board (EOCB) is frequently used instead of the conventional PCB. At this time, the EOCB is formed by inserting an optical waveguide for transmitting and receiving optical signals and comprising polymer and glass fiber, into the PCB.
In other words, the EOCB is a PCB into which the optical waveguide and a glass plate are inserted after the circuit pattern is already formed on the PCB, and it transmits and receives both electrical signals and optical signals. Additionally, the EOCB functions to convert optical signals into electrical signals so as to store data or process signals in an element mounted on the PCB, and very high-speed data telecommunication is realized when the electrical signal is interfaced with the optical signal in the PCB.
Technologies using the above EOCB are applied to switches of a telecommunication network, transmitting and receiving equipment, switches and servers of data communication equipment, communications equipment for the aerospace and avionics industry, mobile telephone base stations of a universal mobile telecommunications system (UMTS), and a backplanes and daughter boards of high-speed Mainframes and supercomputers.
Additionally, in accordance with the recent trend of rapid increasing use of the Internet and improvements in internet service quality, the amount of data to be processed and transmitted has rapidly increased, so it is necessary to expand bandwidth and rapidly process signals. Accordingly, it is increasingly required to use the EOCB as the medium of optical interfacing. That is to say, the electrical signals used in a conventional PCB are undesirably affected by electromagnetic susceptibility (EMS) characteristics during a high-speed switching process on a GHz band, so there remains a need to adopt optical interfacing which is not limited by the EMS characteristics.
However, although it has been ten years or more since the EOCB was first proposed, a desirable methods has not yet been suggested by which elements, the optical waveguide, and the glass fiber are inserted into the multi-layered PCB. In the history of EOCB technology, there have been three types of EOCB: the first and second EOCBs in which the backplane processes signals using the glass fiber in a point-to-point manner, and the third EOCB in which optical signal interfacing is realized using a multi-channel manner so as to simultaneously process a great quantity of data.
Further, a conventional multi-layered PCB is disadvantageous in that when the optical waveguide is heated so as to combine the optical waveguide with the multi-layered PCB, prepreg consisting of a thermally curable epoxy resin is melted and covers the optical signal entrance of the optical waveguide. In other words, the prepreg covers and blocks the optical signal entrance of the optical waveguide, obstructing transmission of the optical signal.